A semiconductor device or a liquid crystal display has been conventionally manufactured in high clean space. To produce such high clean space, a porous film (which will be hereinafter referred to as a porous PTFE film) made of polytetrafluoroethylene (which will be hereinafter referred to as PTFE) is used as a dust collecting filter. A porous PTFE film, which has high dust collection efficiency compared with a filter medium made of glass fiber at the same pressure loss, is favorably used especially for a HEPA filter (High Efficiency Particulate Air Filter) or a ULPA filter (Ultra Low Penetration Air Filter).
On the other hand, clogging due to dust occurs frequently in a porous PTFE film, which has a fiber structure denser than that of a glass fiber filter medium that has been used conventionally, and pressure loss of an air filter unit increases in a short time when the air filter unit is used in an environment, which gives a heavy dust load, such as an outside air processing unit.
With regard to the above problem, a filter medium for an air filter is known, which can prevent clogging due to collected dust and suppress a rise in pressure loss JP 2000-300921 A. The filter medium for an air filter includes a porous PTFE film and an air-permeable porous material made of fiber, and the air-permeable porous material made of fiber is located at an upstream side of the porous film in an air flow. The air-permeable porous material made of fiber has a fiber diameter within the range of 1 to 15 μm, porosity more than or equal to 70%, and basis weight more than or equal to 60 g/m2.
An air intake fitter medium for a turbine is also known, which is a filter medium for a filter wherein a rise in pressure loss is suppressed JP 2002-370009 A. The filter medium includes a porous polytetrafluoroethylene film and an air-permeable support member. Used for the air-permeable support member is fiber having a wide range of a fiber diameter of 0.2 to 15 μm, inclusive.